The present invention relates generally to a marine mud motor drive assembly, particularly useful as part of the drive shaft mechanism in a marsh motor on a shallow water boat.
A marine mud motor is a specialized marine motor used to propel a boat in shallow water applications. These motors are useful in marshes or other shallow water areas where the propeller frequently comes in contact with rocks, mud, logs, and weeds, etc., at the bottom of the body of water. Because these motors operate in shallow water, near the bottom of the body of water, the water they operate in often contains a great deal of dirt, sand or other particulate matter. This water that contains suspended matter creates an environment that is more erosive to moving motor parts than a clean water environment. The motor and drive shaft are designed to propel boats in extreme conditions by allowing the propeller to ride gently over these obstacles with minimal damage to the drive.
A marine mud motor, illustrated in FIG. 1, includes a frame 8 that supports an engine 14. The frame is mounted on a boat 10 by an engine mount 30. A long drive tube 16 is coupled to the frame by supports 28 and the drive tube contains a drive shaft 26. A propeller 22 is connected to the drive shaft and moves the boat forward. This design has been used for more than 30 years and is in use throughout the U.S. and Asia today to propel boats through adverse shallow water conditions.
The conventional design described above has a long drive tube 16 that encases a drive shaft 26 which is three to seven feet in length. This design includes a drive shaft, which is supported during rotation by bronze or composite bushings pressed into the drive tube, one on the bottom end and one on the top end of the drive tube. The bushings are contained in the drive tube assembly 16, which is generally filled with lubrication or grease. Marine mud motors using bronze or composite bushings pressed into the drive tube have been in production around the world for over 30 years.
As with any device operated under water, moisture is a serious problem. Seals are used to keep water out and lubricants inside the drive. Because bronze and composite bushings do not corrode easily, they have conventionally been used as a wear surface in the drive. Roller bearings used in the environment have repeatedly failed due to corrosion and alignment difficulties associated with this conventional design. In addition to the problems caused by corrosion due to moisture, the environment in which marine mud motors operate increases the wear on moving parts due the high level of dirt or silt suspended in the water. The wear caused by dirt, sand or other particulate contained in the muddy water compounds the corrosion problems caused by moisture. The use of roller bearings has failed due to the extreme conditions in which the motors are used, and the eventual intrusion of water, dirt, sand or other particulate, which quickly rusts and seizes the bearing, leading to extremely early failure.
To avoid this early drive shaft failure, multiple seals conventionally are used and the drive tube is filled completely with grease. Even with multiple seals and filling the tube with grease, water, dirt, and particles enter the drive and cause early bearing failure. This early bearing failure usually takes less than two years, even with constant attention and lubrication. For example, users must frequently fill the drive tube with grease, which tends to leak out.
Another significant problem is due to the drives"" inherent design. The rotating shaft causes a Venturi effect which draws water up inside the drive tube past the multiple seals. Because the drive is in frequent contact with the bottom of the body of water, silt and sand accompany the water, which is pulled into the drive tube. Of course, the silt, particles and sand significantly accelerate bushing and drive tube wear and induce early failure.
Even more water and silt is pulled into the drive tube when the drive is at rest in the water. This is caused by cooling parts that create a vacuum, which draws water inside the drive. This additional water, silt and sand further contribute to the problem caused by the Venturi effect.
Because of these problems, bronze bushings, composites and even ceramics have been used as a wear surface for the rotating drive shaft. Other bearings have been tried, but the sand, dirt and water wear them out in a very short time. Accordingly, the use of bronze or composite bushings in the drive tube has been the industry standard.
Conventional drive tubes are completely, or nearly completely, filled with grease which lubricates both the top and bottom drive bushings. This lubrication helps combat any moisture that enters the system and helps to hydraulically balance the long drive shaft, which tends to vibrate during operation due to its length. Seals mounted on each end of the drive tube retain the grease inside the tube and prevent contaminants from entering the drive tube. The lower end of the drive tube supports the propeller and commonly features multiple seals, usually three. The seals are positioned in the conventional direction, that is with the lip of the seal toward the medium to be contained. One of the seals is positioned with the lip toward the grease inside the drive tube, to retain the grease inside the drive tube. One or two of the three seals are positioned with the lips toward the water, in an effort to prevent water from entering the drive tube. Multiple seals are used because the long drive tube causes a Venturi effect, which attempts to draw water inside the drive tube. The outer two seals slow this xe2x80x9cpumpingxe2x80x9d process down. Regardless, the Venturi effect is strong and eventually small amounts of water with silt enter the system and cause accelerated wear and contamination of the lubricant. For this reason, bearings have not been used successfully because the moisture and dirt causes early bearing failure. Thus, only bushings have been used for the past 30-plus years.
Traditional bushings of almost any thickness can be used, however bushings with a thickness of approximately xe2x85x9 of an inch are usually selected. By selecting bushings that are relatively thin, the size of the drive tube can be minimized. Conventional drive tubes have an inside diameter only slightly larger than the outside diameter of the drive shaft. This is desirable for a number of reasons. For instance, the gap between the tube and the shaft is completely, or nearly completely, filled with grease. Increasing the gap may necessitate an increase in the amount of grease used, which may also increase the costs of maintaining the drive assembly.
In addition to concerns described above, the components of mud motors are designed to be as light as possible in order to minimize fuel consumption and loss of power. By maintaining a relatively small diameter tube and drive shaft, the weight of the drive assembly can be minimized. Increasing the diameter of the drive tube also increases the surface area of the tube in contact with the water. Because the end of the drive tube near the propeller is submerged in water, the water exerts an upward force on the drive tube as the boat travels through the water. This upward force can be considerable, and if the drive has sufficient surface area, it may force the drive tube out of the water. This can cause the drive tube to hydroplane, or xe2x80x9cflyxe2x80x9d out of the water. By maintaining a relatively small drive tube, with a correspondingly small surface area, the lift is reduced as the boat travels through the water.
It has been recognized that it would be advantageous to develop a system which would allow bearings to be used in a marine motor drive shaft. In addition, it would be valuable to have a device which can overcome the Venturi effect and stop the flow of water, silt and sand into a drive shaft.
The present invention provides a drive assembly for a marine mud motor with an elongated drive tube containing a lubricant. The drive tube rotatably receives a drive shaft including a propeller on the drive shaft end. The drive assembly comprises an expanded drive assembly housing having an upper end, lower end, and housing cavity. The inside diameter of the expanded drive assembly housing is larger than the inside diameter of the drive tube, allowing for selection of bearings and/or seals of a larger diameter than the drive tube, so that a conventional drive tube can be used with larger sized bearings. An outer seal is mounted in the lower end of the drive assembly housing and structured to stop lubricant flow from the housing cavity. An inner seal is provided, between the outer seal and the upper end of the drive assembly housing, and spaced apart from the outer seal to provide an area for pressurized lubricant.
In accordance with one aspect of the present invention, the system includes a drive assembly containing a roller bearing and a structured seal arrangement, enclosed in an expanded assembly housing. The roller bearing drive assembly provides a drive rotation pressure which overcomes the system""s inherent Venturi effect by first reversing the Venturi effect, and then creating a pressurized lubricant chamber. The drive assembly contains a bearing set and two or more seals. Two seals can develop the desired results, although more than one bearing or two seals may be used. Hydraulic pressure is generated on the lower end of the bearing due to its accelerated rotation and slope. This pressure is used to force lubrication or grease past the lip of the innermost seal, which is installed in a reverse direction. The grease then flows under pressure into a chamber between the inner and outer seals. The pressurized lubricant in the lubricant chamber cannot escape because the two seals are installed in opposite directions, allowing pressurized lubricant to enter from the bearing side but not escape the area between the two seals. The developed pressure reverses the normal tendency of the drive to create a Venturi effect that attempts to draw water inside the drive tube past seals into the drive tube which leads to increased wear, corrosion, and early failure. Alone, this pressure helps keep water out of the system, but when captured in a seal assembly, a secondary barrier is provided that maximizes its sealing capability, thus extending the life of the drive.
This configuration allows the use of roller bearings in mud motors that could not use roller bearings as a wear surface due to water contamination. The use of an integral assembly having a removable housing, containing at least one bearing, at least one seal, and seal cap benefits this application. By utilizing an expanded housing, the present invention can be used with smaller drive tubes and drive shafts, regardless of the outside diameter of the bearings and seals selected. The present invention can also be advantageously used on the upper end of a mud motor drive tube, where the tube and drive shaft are attached to the motor of the boat.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate by way of example, the features of the invention.